Extended surface area substrate particles, such as activated carbon, alumina, zeolites, and the like, are widely used in air filtration because of the ability of such materials to remove a wide range of different materials. The filtration characteristics of these materials arises from a highly porous or convoluted surface structure. In the case of activated carbon, the surface porosity results from controlled oxidation during the “activation” stage of manufacture. Activated carbon has been used for air filtration for many decades.
The ability of the carbon to remove a contaminant from the air by direct adsorption depends on a molecular-scale interaction between a gaseous molecule and the carbon surface. The extent of this interaction depends upon factors that include the physical and chemical surface characteristics of the carbon, the molecular shape and size of the gaseous compound, the concentration of the gaseous compound in the gas stream to be filtered, residence time in the carbon bed, temperature, pressure, and the presence of other chemicals. As a rule of thumb, for a single contaminant, the extent of adsorption is primarily dependent on boiling point. In general, the higher the boiling point, the greater the capacity of carbon to remove the chemical.
Accordingly, carbon does not have a great capacity by itself to remove lower boiling point gases. Treatments have been devised in which chemicals are coated on the carbon to provide filtering capabilities towards lower boiling point gases. These treatments are generally known as “impregnation” methods, and the result of treatment is an “impregnated” carbon.
Over the course of this century, development of impregnation techniques has progressed so that a variety of impregnants are available for removing a wide range of different chemicals. Progress has been accelerated during wartime, when actual and perceived threats spurred the development of specialized carbons. However, there has hitherto been a distinction between the types of filter media particles used for military applications, and those used in industrial applications. Military requirements have made it necessary for filter media particles to be capable of removing a range of chemicals, and so multi-component impregnation formulations have been devised. In industry, where the nature of hazards is known in advance, the practice has been to select a filter appropriate to the known hazard. Consequently, filters with capability toward a specific type of chemical or class of chemicals have developed for industrial applications.
Over time, regulatory structures for the selection and use of respiratory protective equipment have evolved, along with approvals systems to ensure that designs of equipment on the market are capable of meeting necessary performance requirements. Such approvals systems have been generated for industrial purposes across international boundaries. These include the European Norm system that is adopted widely in Europe and elsewhere in the world. Another example are the approvals requirements of the US National Institute for Occupational Safety and Health that have been adopted in the USA, Canada and certain other countries. For military requirements, performance specifications are determined by each national need, although there are some internationally agreed upon standards under the North Atlantic Treaty Organisation.
The first U.S. patent for a treatment of carbon to remove a variety of military gases derived from developments to protect personnel in World War I battles in which chemical agents were used in excess. The patent by Joshua C. Whetzel and R. E. Wilson (US Patent 1,519,470, 1924) described the use of an ammoniacal solution of copper carbonate to impregnate a granular activated carbon. This technique became known as “Whetlerization”, and the carbon product “Whetlerite”. Variations on this technique have been developed over time. (U.S. Pat. Nos. 2,920,050, 2,920,051, DE 1,098,579, FR 1,605,363, JP 7384,984, CZ 149,995).
During World War II, substantial technical investigations were made into the use of impregnated carbons. The U.S. research in this area is summarized in “Military Problems with Aerosols and Nonpersistent Gases”, Chapter 4: “Impregnation of Charcoal”, by Grabenstetter, R. J., and Blacet, F. E., Division 10 Report of US National Defense Research Committee (1946) pp.40–87. This report provides in depth coverage of a number of impregnant formulations.
The United Kingdom pursued a slightly different impregnation approach. There, copper oxide was mixed with coal prior to carbonization and activation, so that the activated carbon contained metallic copper distributed throughout its structure. This material was the basis for the filter carbons used in World War II.
The ability of the carbon to remove cyanogen chloride (CK) was improved by the application of the amine pyridine or, separately, by impregnation with chromium in the form of sodium dichromate. This form of carbon, in combination with a pyridine impregnant, was used in military respirator filters manufactured in the 1970s.
Post World War II research has explored how the addition of organic compounds to impregnated carbon could improve the shelf life. Experiments were undertaken in the UK, France and elsewhere with various amines. One such material found to improve the shelf life towards cyanogen chloride is triethylenediamine (also known by other names such as TEDA, DABCO, or 1,4-diazabicyclo-2,2,2-octane). When impregnated on carbon, TEDA has been found in its own right to be capable of reacting directly with cyanogen chloride and is also highly capable of removing methyl bromide and methyl iodide. TEDA is strongly adsorbed onto carbon, is stable, is effective at low levels, and has minimal toxicity compared with other amine compounds. TEDA is a solid at room temperature, but sublimes readily.
Chromium has traditionally been used as a carbon impregnant in military applications, as it facilitates the satisfactory removal of hydrogen cyanide and cyanogen chloride (CK). Because the hexavalent ionic form of chromium has been identified as a potential lung carcinogen, work undertaken in recent times and dating back to the early 1970's has explored formulations that avoid or reduce the level of chromate salts as impregnants.
In recent times, the traditional role of military forces has changed from a more or less predictable battlefield conflict to encompass peace-making and peace-keeping roles, and supporting civilian authorities in emergency response. Such activities may involve responding to the release of chemicals by accident or intent. Intentional release of chemicals, referred to as “chemical terrorism”, has occurred in fact and been threatened numerous times. These incidents may involve chemicals that have been traditionally regarded as military threats or may involve hazardous chemicals normally used in industry. The response to these hazards is ultimately likely to involve both civilian and military authorities and is likely to require protection systems that meet industrial approvals as well as military performance requirements.
Filtration-based protection systems are appropriate for personnel undertaking various tasks at some distance from a point of chemical release. For such cases, it is most desirable to be able to respond to a hazard quickly and without delay. Conventionally, however, delay may be inevitable as it may be necessary to first identify a threat in order to select an appropriate filter. In order to be able to respond to a wide range of possible hazards, it has been necessary to carry inventories of many different kinds of filters. It would be much more desirable to have one filter type that can provide protection against many different hazards. Such a multi-purpose filter desirably would accommodate both industrial and military needs.
U.S. Pat. No. 5,492,882 describes filter medium particles impregnated with Cu compounds, Zn compounds, ammonium sulfate, moisture, molybdenum compounds, and other agents in some embodiments. The molybdenum compounds tend to be strongly acidic and are used to help protect against not just HCN and cyanogen chloride (CK), but also basic gases such as ammonia. However, Mo is used in these formulations at relatively high weight loadings, occupying greater portions of the substrate particles that might otherwise be available for other impregnants were Mo to be efficacious and/or used at lower loadings. Mo compounds tend to be strongly acidic, limiting formulation flexibility to co-impregnants whose functionality is not unduly impaired by the acidic character of Mo compounds. Mo is also quite expensive by comparison.
Yet, merely reducing or eliminating the Mo in a formulation is not a simple task. The significant challenge is to find a way to reduce or eliminate Mo while still providing protection against not just HCN and CK, but also ammonia and other basic contaminants.